The present invention relates to the manufacture of ceramic matrix composites which comprise reinforcing fibres in a ceramic matrix, particularly ceramic fibres in a ceramic matrix.
It is known to produce ceramic matrix composites by chemical vapour infiltration, by directed metal oxidation or by sol-gel processes.
The chemical vapour infiltration method comprises forming a fibre preform and then depositing a ceramic matrix on and between the fibres in the fibre preform. The ceramic matrix is deposited by decomposing a chemical compound in the vapour form and depositing the resulting ceramic onto the fibre preform. The chemical vapour infiltration method is a very expensive method and requires expensive apparatus, furthermore in some instances the chemical compounds which are to be decomposed are toxic. The chemical vapour infiltration method has to be performed at a relatively high temperature to decompose the chemical compound.
The directed metal oxidation method comprises forming a fibre preform and then growing a ceramic matrix on and between the fibres in the fibre preform. The ceramic matrix is grown by placing the fibre preform on the surface of a molten metal and oxidising the metal such that the metal oxide grows into the fibre preform. The directed metal oxidation method is also a very expensive method and requires expensive apparatus. Additionally there is always some unreacted metal which is difficult to remove. The directed metal oxidation method also has to be performed at a relatively high temperature to melt the metal.
The sol-gel method comprises either vacuum impregnation or filament winding. The filament winding method comprises passing each fibre through a container of the sol, winding the impregnated fibre on a mandrel of the desired shape, converting the sol to a gel and then heating to convert the gel to a ceramic matrix. The sol-gel method is cheaper than the chemical vapour infiltration method and the directed metal oxidation method and also is performed at a relatively low temperature. The sol-gel method has a low yield and has large shrinkage of the ceramic matrix resulting in cracking of the ceramic matrix. Also multiple infiltration and densification cycles are required.
The present invention seeks to provide a novel method of manufacturing a ceramic matrix composite.
Accordingly the present invention provides a method of manufacturing a ceramic matrix composite comprising the steps of:
(a) forming a slurry comprising a ceramic sol, filler material and a suspension medium,
(b) forming a plurality of laminates of fibres,
(c) applying the slurry to each of the plurality of laminates of fibres,
(d) stacking the plurality of laminates of fibres on a mould,
(e) applying pressure to the stack of laminates of fibres to remove the suspension medium from the slurry to solidify the ceramic sol and thereby produce a ceramic matrix composite.
Preferably step (e) comprises covering the stack with a porous membrane, covering the porous membrane with a breather fabric, covering the breather fabric with a vacuum bag and evacuating the vacuum bag to remove the suspension medium from the slurry to solidify the ceramic sol and thereby produce a ceramic matrix composite.
Preferably the vacuum bag is evacuated to a pressure less than 3000 Pa and held at that pressure for about 10 hours.
Preferably the method comprises heating the stack during or after evacuation of the vacuum bag to encourage the solidification of the ceramic sol. Preferably the stack is heated to a temperature in the range 60xc2x0 C. to 150xc2x0 C.
Preferably the method comprises pressure less sintering after evacuation of the vacuum bag.
Preferably the ceramic sol comprises silica, alumina or mullite particles and the filler material comprises silica, alumina or mullite particles. The filler material particles may have a diameter greater than 1 micron and the ceramic sol particles may have a diameter of about 40 nanometers.
Preferably the fibres comprise silica, alumina, mullite or a mixture of any two.
Preferably the mould is hollow and has an inner surface, the method comprising stacking the laminates of fibres on the inner surface of the hollow mould.
The mould may comprise a styrofoam mould. The mould may define an aerofoil shape.
The fibres may comprise mullite or mullite and alumina, the ceramic sol comprises silica and the filler material comprises alumina.